Reaching movements performed without vision of the moving limb are continuously monitored, during their execution, by feedback loops (designated nonvisual). In this study, we investigated the functional anatomy of these nonvisual loops using positron emission tomography (PET). Seven subjects had to "look at" (eye) or "look and point to" (eye-arm) visual targets whose location either remained stationary or changed undetectably during the ocular saccade (when vision is suppressed). Slightly changing the target location during gaze shift causes an increase in the amount of correction to be generated. Functional anatomy of nonvisual feedback loops was identified by comparing the reaching condition involving large corrections (jump) with the reaching condition involving small corrections (stationary), after subtracting the activations associated with saccadic movements and hand movement planning [(eye-arm-jumping minus eye-jumping) minus (eye-arm-stationary minus eye-stationary)]. Behavioral data confirmed that the subjects were both accurate at reaching to the stationary targets and able to update their movement smoothly and early in response to the target jump. PET difference images showed that these corrections were mediated by a restricted network involving the left posterior parietal cortex, the right anterior intermediate cerebellum, and the left primary motor cortex. These results are consistent with our knowledge of the functional properties of these areas and more generally with models emphasizing parietal-cerebellar circuits for processing a dynamic motor error signal.

Whenever the head turns, the vestibuloocular reflex (VOR) produces compensatory eye movements to help stabilize the image of the visual world on the retina. Uncompensated slip of the visual world across the retina results in a gradual change in VOR gain to minimize the image motion. VOR gain changes naturally during normal development and during recovery from neuronal damage. We ask here whether visual slip is necessary for the development of the chicken VOR (as in other species) and whether it is required for the recovery of the VOR after hair cell loss and regeneration. In the first experiment, chickens were reared under stroboscopic illumination, which eliminated visual slip. The horizontal and vertical VORs (h- and vVORs) were measured at different ages and compared with those of chickens reared in normal light. Strobe-rearing prevented the normal development of both h- and vVORs. After 8 wk of strobe-rearing, 3 days of exposure to normal light caused the VORs to recover partially but not to normal values. In the second experiment, 1-wk-old chicks were treated with streptomycin, which destroys most vestibular hair cells and reduces hVOR gain to zero. In birds, vestibular hair cells regenerate so that after 8 wk in normal illumination they appear normal and hVOR gain returns to values that are normal for birds of that age. The treated birds in this study recovered in either normal or stroboscopic illumination. Their hVOR and vVOR and vestibulocollic reflexes (VCR) were measured and compared with those of untreated, age-matched controls at 8 wk posthatch, when hair cell regeneration is known to be complete. As in previous studies, the gain of the VOR decreased immediately to zero after streptomycin treatment. After 8 wk of recovery under normal light, the hVOR was normal, but vVOR gain was less than normal. After 8 wk of recovery under stroboscopic illumination, hVOR gain was less than normal at all frequencies. VCR recovery was not affected by the strobe environment. When streptomycin-treated, strobe-recovered birds were then placed in normal light for 2 days, hVOR gain returned to normal. Taken together, the results of these experiments suggest that continuous visual feedback can adjust VOR gain. In the absence of appropriate visual stimuli, however, there is a default VOR gain and phase to which birds recover or revert, regardless of age. Thus an 8-wk-old chicken raised in a strobe environment from hatch would have the same gain as a streptomycin-treated chicken that recovers in a strobe environment.